No known rocks have survived from the first 500 million years of Earth history, but studies of single zircons suggest that some continental crust formed as early as 4.4 Ga, 160 m.y. after accretion of the Earth, and that surface temperatures were low enough for liquid water. Surface temperatures are inferred from high d18O values of zircons. The range of d18O values is constant throughout the Archean (4.4-2.6 Ga) suggesting uniformity of processes and conditions. The hypothesis of a Cool Early Earth suggests long intervals of relatively temperate surface conditions from 4.4 to 4.0 Ga that were conducive to liquid-water oceans and possibly life. Meteorite impacts during this period may have been less frequent than previously thought.

Figure 1. Crystallization age (U-Pb) and oxygen isotope ratio (d18O) for Archean magmatic zircons. Distribution of magmaticd18O values does not change throughout the Archean. Most magmas had a primitived18O value similar to that in the mantle today ("mantle zircon"), but some zircon values are as high as 7.5‰. High-d18O zircons and host magmas resulted from melting of protoliths that were altered by interaction with liquid water at low temperatures near surface of Earth (see text). Timeline (inset, lower right) shows: (1) accretion of the Earth, (2) formation of the Moon and the Earth’s core, (3) minimum age of liquid water based on highd18O zircon, (4) Acasta gneiss, and (5) Isua metasedimentary rocks. (Valley et al. 2002)

Figure 2. Histograms of d18O. A: Olivine from mantle xenoliths and Hawaiian basalts. B: Zircon xenocrysts from kimberlites in S. Africa. C: Zircons from igneous rocks of Superior province, Canada. D: Ion microprobe analyses of single zircons from Jack Hills, Western Australia. The Jack Hills zircons (D) are higher in d18O than the mantle. Such high d18O values indicate that the protolith of granitic magmas experienced low temperature interaction with liquid water in a near surface environment (Valley et al. 2002)

Figure 3. Estimates of meteorite impact rate for first two billion years of Earth history. Two hypotheses are shown: exponential decay of impact rate (dashes, and a cool early Earth/ late heavy bombardment (solid curve, this study). In either model, spikes occurred owing to isolated large impacts. Evidence for liquid water comes from high-d18O zircons (>4.4 Ga to >4.0 Ga) and sedimentary rocks (Isua 3.8-3.6 Ga). The cool early Earth hypothesis (solid curve) suggests that impact rates had dropped precipitously by 4.4 Ga, consistent with relatively cool conditions and liquid water on the surface of the Earth. (Valley et al. 2002)

Figure 4. Artist’s rendering of a Cool Early Earth 4.4 billion years ago. Recent evidence from single crystals of zircon suggests that surface temperatures were relatively low and that liquid water would have formed oceans rather than a thick steam-rich atmosphere (Valley et al. 2002). Such oceans could have promoted the evolution of life. The hypothesis of a Cool Early Earth contrasts with earlier ideas that magma covered the Earth, which lead to the first 500 million years of Earth history being named "Hadean" (hell-like). (Graphic: Andrée Valley and Mary Diman)